Bio-assisted heap leaching for recovery of base metals is only carried out commercially on secondary copper sulphide ores. Recent work in Australia has seen the introduction of heap leaching for recovery of nickel from nickel sulphide ores on a semi-commercial test basis(1). Bio-assisted heap oxidation of refractory gold ores is also used as a pre-treatment process for recovery of gold from such ores.
Typically the secondary copper sulphide heaps operate at temperatures in the range of 10° C. to 25° C. and rely on the exothermic oxidation of secondary copper sulphide minerals to keep the temperature of the heap above ambient conditions. The relatively low temperature limits the rate of sulphide mineral oxidation that can be achieved. Additionally, chalcopyrite ores cannot be leached at these low temperatures because chalcopyrite is generally considered to be refractory to leaching at such temperatures.
An increase in the operating temperature of existing and new sulphide heap leach operations would significantly reduce leach times; ore and metal inventory, ultimate metal extraction and enable the leaching of copper from chalcopyrite ores.
It is well known that the mineral chalcopyrite can be leached satisfactorily at higher temperatures (between approximately 60° C. and 90° C.) using thermophilic microorganisms. Chalcopyrite bearing flotation concentrates (treated in stirred tank bioleach reactors) and chalcopyrite-bearing ores (treated in heap leach test columns) have both been successfully processed in the laboratory. Mintek claim a process whereby chalcopyrite concentrates can be bioleached using moderate themnophiles at around 45° C. by a combination of ultra fine milling and redox control of the slurry as indicated in WO 01/31072 A1. The metal extraction from chalcopyrite ores achieved in simulated heap leaching, using laboratory columns, is dependent on the particle size of the ore, finer sizes generally increasing the amount of mineral that is accessible to the lixiviant. Commercial attempts to use bio-assisted heap leaching on chalcopyrite ores have failed (copper recoveries typically much less than 50% in long time periods ˜1-10 years) primarily because of being unable to maintain the ore temperature at that required to satisfactorily leach chalcopyrite. Although chalcopyrite ores can be processed technically by crushing, milling, flotation of a concentrate and processing of the concentrate by a hydrometallurgical process or smelting, these steps are all relatively expensive and heap leaching would provide a more cost effective solution. Furthermore, many primary copper ores are too low in grade to be economic using conventional processes, but heap leaching may make them economically viable, thus opening many copper deposits to be treatable.
In addition to being able to treat chalcopyrite ores, higher temperatures in the heap leaching of base metals from sulphide ores would result in significantly higher leaching rates. In turn, leach pad area as well as ore and metal inventory would be greatly reduced giving substantial economic benefit. For example, a mine treating 15 Mtpa ore with a grade of 1% copper and an overall copper recovery of 80% (producing 120 ktpa copper) would typically have an area under leach of at least 1.5 million m2. Halving this area (by reducing the leach cycle by a factor of 2) would save US$10-30 million on pad construction costs, as well as a similar amount via a reduction in working capital. Higher temperatures are also likely to increase the ultimate amount of metal extracted from the ore (compared to operating at a lower temperature) and ultimate metal extraction is perhaps generally one of the most important factors effecting economic performance. In the previous example an extraction increase of just 5% increases the project NPV, at a 10% discount rate, by about US$60 million. Additionally at higher temperatures pyrite will be oxidised and generate sulphuric acid, reducing the amount of fresh sulphuric acid added into the ore. A reduction in acid consumption of 1 kg/t ore, with an acid price of US$50/t in the example would yield savings of US$0.75 million per annum.
Chalcopyrite ores may contain other sulphide minerals in addition to chalcopyrite, for example covellite, chalcocite, bomite, enargite and pyrite. The oxidation of these sulphide minerals is exothermic in nature; on average sulphide minerals have a calorific value of around 25000 kJ/kg of sulphide. The rate of oxidation of these minerals determines how quickly this energy is released. If we consider 1000 m3 of ore with a bulk density of 1.7 t/m3, the ore mass is 1700 t. Consider, as an example, that the ore contains 2% sulphide in the form of sulphide minerals. If the sulphides are fully oxidised over a period of 183 days, then the energy (1700 t×1000 kg×2% sulphide×25000 kJ/kg=850 GJ or 236 MWh) is released over a period of 4368 hours. In this example the power generation represents ˜54 W/m3 and can be likened to an electric light bulb, uniformly distributing ˜54 W within each cubic meter of the ore heap. Furthermore the energy required to heat the rock (ignoring for the moment any moisture and air within the heap), with a specific heat of ˜1000 J/kg/° C. from ambient (say 20° C.) to the operating temperature (say 60° C.) is 1700 t×1000 kg×˜1000 J/kg/° C.×(60° C.−20° C.)=68 GJ; this energy is far less than that which is released during the oxidation reaction. Superficially, therefore, the energy released during the oxidation process is more than adequate to heat up the heap. In reality of course the moisture within the heap, as well as any air, has also to be heated, but this requirement is quite small compared to the requirement for the ore.
However there are several problems with this simplistic approach. There are various heat losses to take into account. In the case of base metals, for example copper, bio-assisted heap leaching requires the removal of dissolved copper and the ore heap is typically irrigated with an acidic lixiviant (usually solvent extraction raffinate) containing at least some iron in solution. The incoming irrigation solution will always be cooler than a satisfactorily operating heap and absorb energy (released in the exothermic oxidation process) as it passes down the heap, increasing in temperature as it does so. Bio-assisted heap leaching also requires air to be blown through the heap to provide oxygen for the oxidation reactions. In typical Chilean conditions, for example, the ambient air will be cold and contain a small amount of moisture. As the air enters the heap it meets the hot solution and cools the solution down transferring energy to the gas phase, which becomes warmer and more humid as it continues to rise, passing counter-current to the solution, until it meets the cooler regions at the top of the heap, where it cools down and water is condensed from the air. Additionally there will be some heating (during daylight) and cooling (during the night) of the heap surface, as well as cooling due to surface evaporation and radiative emission. At the same time the sulphides in the rock are generating heat of reaction; a complex system that will nevertheless reach a dynamic operating temperature profile throughout the depth of the heap. There will be very minor losses from the sides and bottom of the heap as well. Surface heat losses (as well as water to evaporation) can be reduced by partially covering the heap with an insulating layer. Plastic sheeting has been used recently in industrial operations.
The air is provided on commercial operations by blowing air through a network of perforated pipes beneath the heap. However the air has another important role, over and above the provision of oxygen for the oxidation reactions, which is the movement of heat up or down within the heap.
Dixon gives a good account of the overall energy balance of the heap system(2). Of particular note in Dixon's findings was the importance of the aeration mass flow Ga, relative to the solution irrigation mass flow Gl, both expressed in kg per m2 per hour. In the system considered by Dixon, there was a ratio of the aeration rate to the irrigation rate, that is to say Ga/Gl, which produced the highest heap average temperature, which Dixon found to be ˜38° C. at a Ga/Gl ratio of 0.5. Below this ratio, the average heap temperature fell off. Dixon indicated there was a critical blowing rate, corresponding to Ga/Gl ratio of 0.35, for heat to be begin moving towards the top of the heap.
Dixon explained the net heat movement effect by the “combined advection coefficient”. The net or combined advection coefficient is a measure of the transfer of energy up or down the heap as a result of the transport of solution and gas phases up or down the heap. The solution moves heat downwards and moist warm air moves it upwards.
Dixon does not recommend a method of controlling the heap. However, he concluded that increasing the temperature within a heap could be achieved by:                Choosing a lower value for the solution irrigation rate than the current practice                    Higher rates of irrigation result in washing of heat out to the PLS, to the detriment of heap temperature.                        Choosing a higher value for the aeration rate than the current practice                    The aeration rates in typical industrial operations are insufficient to move heat upwards; consequently heat is lost to the irrigation solution as it exits the heap. Increasing the aeration rate can push heat upwards into the heap, significantly increasing average heap temperature. Increasing aeration rates was one of Dixon's major recommendations.                        Applying an evaporation shield to the heap surface                    An evaporation shield will reduce the effective surface heat transfer coefficient and resulting heat losses, increasing the average heap temperature. Recent industrial practice has seen heaps being covered with plastic sheeting.                        Heating the irrigation solution                    Due to the day/night cycle and evaporative cooling at the top of the heap, Dixon found that heating the irrigation solution from 10 to 30° C. only increased the average heap temperature by about 3° C., in the best case (which was co-incident with operating the heap sub-optimally) and barely had any effect at all when operating the heap at a more favourable Ga/Gl ratio. Covering the heap with plastic sheeting only mitigates this effect to a small extent.                        Heating the air with and without humidification.                    Heating air without humidifying it has little effect because of the low heat capacity of dry air. Humidifying and heating the air significantly increases heap temperature, but at the cost of applying external energy.                        
Further results were presented by Dixon(3) for the application of his recommendations to the operation of a Geobiotics heap, in which sulphide concentrates, coated onto a support rock, are leached on a heap. From this work he concluded that the irrigation rate required was between 5 and 10 kg/m2/hr and that the aeration rates were between 25 and 40% of the irrigation rate.
AU-A-60837/90 (“Oxidation of Mineral Heaps”) claims a method for controlling the rate of oxidation in the heap based on the use of the mathematical model to predict the effect of the changes in the operating variables. These control variables are heat and water. AU-A-60837/90 describes the control of the heap as a process of the determination of the oxidation rate and the mathematical prediction of the effect of a variety of control variables will have on the global and intrinsic oxidation rates at some point in the future. The results of the mathematical prediction are used to choose the values of the control variables in order to control the operation of the heap.
Ritchie(4) summarises the work performed by himself, Pantelis and Davis. This work elaborates on the control philosophy suggested in AU-A-60837/90, that is, to control the temperature in the heap through a measure of the oxidation rate, and choose the values of the control variables based on the predictions of a mathematical model. The oxidation rate can be determined by a number of methods, including the determination of the oxygen in the effluent gas.
In this specification:                The term “take-off” means a point at which the power generated in the heap rises from a fairly low rate to a higher rate in a relatively short time period; it is the system bifurcation point.        The phrase “average irrigation rate” means the total irrigation amount applied to the heap over the total duration of the leach cycle expressed as a average hourly irrigation rate per unit area; and the phrase “average aeration rate” means the total gas amount passing through the heap over the total duration of the leach cycle expressed as a average hourly aeration rate.        The phrase “instantaneous irrigation rate” means the instantaneous irrigation rate applied to the heap over any time period shorter than the total duration of the leach cycle expressed as instantaneous hourly irrigation rate per unit area; and the phrase “instantaneous aeration rate” means the instantaneous gas flow rate applied over any time period shorter than the total duration of the leach cycle expressed as instantaneous hourly aeration rate per unit area.        The terms irrigation rate and aeration rate refer to the instantaneous irrigation rate and the instantaneous aeration rate respectively, unless otherwise stated.        The term “heap leaching” means leaching of ore in heaps or dumps.        The term “oxygen utilization of the heap” means the total oxygen consumed within the heap expressed as a percentage of the total oxygen passed through the heap.        The term advection means the net transfer of energy up or down the heap.        